1. Field of the Invention
The present invention relates to a method for mounting electronic elements, such as IC chips, over the wiring pattern on a circuit board (for example, a multichip module circuit board) by bonding the electronic elements thereto using a thermosetting adhesive such as an anisotropic conductive adhesive film.
2. Description of the Related Art
Electronic devices developed in recent years offer higher performance, notwithstanding their lighter weight and smaller, thinner profiles. An associated trend has been a decrease in the area available on the circuit board for mounting the various electronic elements (capacitor elements, resistor elements, IC chips, etc.) used in such electronic devices. Accordingly, electronic element-to-circuit board connections are currently produced by thermocompression-bonding of the connection terminals of the electronic elements to the wiring pattern of the circuit board by means of a thermosetting adhesive (e.g., an anisotropic conductive adhesive, anisotropic conductive adhesive film, insulating adhesive, or insulating adhesive film). In the case of IC chips, an IC chip in bare chip form is mounted through flip-chip joining thereof, using a thermosetting adhesive, to a circuit board (termed an interposer) of substantially the same dimensions as the chip, or a plurality of IC chips are mounted through flip-chip joining, using a thermosetting adhesive, to a multichip module circuit board.
Flip-chip mounting as practiced currently involves arranging the interposer board or multichip module circuit board on a stage, applying a thermosetting adhesive thereover, positioning the IC chip(s) thereon, and then thermocompression-bonding the IC chip(s) to the interposer or multichip module circuit board by a heating tool.
In the process of checking IC chips to determine if any are defective, the extremely fine pitch of the wiring patterns on IC chips makes it difficult to inspect IC chips per se that are as yet unmounted and in a bare chip form. Accordingly, typical practice is to check IC chips for defects after the IC chips have been mounted onto an interposer or multichip module circuit board, by performing a conductivity check on the IC chips via the circuit board.
If the check should reveal an IC chip to be defective, it will be necessary to repair the defective IC chip, since discarding the entire interposer or multichip module circuit board on which the defective IC chip is mounted would push up production costs. Attempts have been made to accomplish this by depressing the reaction rate of the thermosetting adhesive overall during flip-chip mounting so as to allow the IC chip to be peeled away with relatively low shearing force.
However, the approach of depressing the reaction rate of the thermosetting adhesive overall during flip-chip mounting, at least to the extent that the IC chip can be peeled away with relatively low shearing force possess a problem that the cohesive force of the thermosetting adhesive is lower as a result, and conductivity checking cannot be adequately accomplished.
Conversely, raising the reaction rate of the thermosetting adhesive overall in order to enable the conductivity checking will raise the cohesive force of the thermosetting adhesive, and it will make it difficult to repair a defective IC chip.
With the foregoing in view, it is an object of the present invention to solve the problems associated with the prior art by providing a method for mounting electronic elements, whereby both adequate conductivity checking and repair of electronic elements may be achieved during the process of bonding an electronic element, such as an IC chip, over the wiring pattern on a circuit board (for example, a multichip module circuit board) by bonding thereto with a thermosetting adhesive such as an anisotropic conductive adhesive film.
The inventors perfected the present invention based on the discovery that during thermocompression-bonding of an electronic element to a circuit board by means of a thermosetting adhesive, where the electronic element and the circuit board are subjected to a temporary thermocompression-bonding process conducted in such a way that the thermosetting reaction rate of the thermosetting adhesive at the circuit board-bonding face thereof is lower than that at the electronic element-bonding face thereof, (i) the cohesive force of the thermosetting adhesive at the interface between the circuit board and thermosetting adhesive is relatively low, allowing the electronic element to be peeled away from the circuit board with relatively low shearing force, thus making it possible to repair the electronic element; (ii) the cohesive force of the thermosetting adhesive at the interface between the electronic element and thermosetting adhesive, on the other hand, is relatively higher, making conductivity checking possible; and (iii) after completing the conductivity check, the electronic element can be reliably mounted on the circuit board by fully setting the thermosetting adhesive.
Specifically, the present invention provides a method for mounting electronic elements, comprising the step of arranging a thermosetting adhesive between a circuit board, situated on a stage, and an electronic element to be connected with the wiring pattern of the circuit board, and then bonding the electronic element to the circuit board by the application of heat and pressure to the electronic element-bonding face, wherein the electronic element and the circuit board are subjected to a temporary thermocompression-bonding process conducted in such a way that the thermosetting reaction rate of the thermosetting adhesive at the circuit board-bonding face thereof is lower than reaction rate with thermosetting at the electronic element-bonding face thereof, and the thermosetting adhesive is then set fully in order to bond the electronic element to the circuit board.